Destroyed child still is referenced.

Quick explaination, this script is one that assigns a cursor object to the mouse. On selection of a new cursor object this script is called, where the old cursorobject is deleted and a new one added: it would seem that the problem actually lies with the previous "mousecursor" object, which is getting destroyed, is still being referenced? (see comments in code for explaination on the issue):

 public void SetObjToPlace(GameObject obj)
                  {
                      objToPlace = obj;
                      cursorChildren = GetComponentsInChildren<Transform>();
              
                      // Clear collisionScripts
                      collisionScripts.Clear();
              
                      // Delete old object cursor
                      if (cursorChildren != null)
                      {
                          foreach (Transform child in transform)
                          {
                              Destroy(child.gameObject);
    //  ^ THIS OBJECT STILL GETS REFERENCED IN AddRigidBodyAndTriggerToAllChildren
                          }
                          cursorChildren = GetComponentsInChildren<Transform>();
                      }
              
                      // Create new object cursor
                      GameObject newCursor = Instantiate(obj);
                      newCursor.transform.parent = transform;
              
                      AddRigidBodyAndTriggerToAllChildren(transform);
             // ^ This ^ function throws the error "Can't add component 'Rigidbody' to NameOfPreviouslyDestroyedObject because such a component is already added to the game object (well no shit, but it shouldn't check that destroyed child)
              
                      transform.rotation = Quaternion.identity;
                      newCursor.transform.position = transform.position;
                      SetChildShader();
                  }
         
             private void AddRigidBodyAndTriggerToAllChildren(Transform parent)
                 {
                     foreach (Transform child in parent)
                     {
                         BoxCollider[] childColliders = child.gameObject.GetComponents<BoxCollider>();
                         if (childColliders.Length > 0)
                         {
                             Rigidbody childRB = child.gameObject.AddComponent<Rigidbody>();
         // ^ This is the line throwing the error ^
                             childRB.useGravity = false;
         // ^ Causing a nullref for childRB here 
                             childRB.isKinematic = true;
             
                             PlacementCollision collisionScript = child.gameObject.AddComponent<PlacementCollision>();
                             collisionScripts.Add(collisionScript);
             
                             foreach (BoxCollider collider in childColliders)
                             {
                                 collider.isTrigger = true;
                             }
                         }
             
                         AddRigidBodyAndTriggerToAllChildren(child);
                     }
                 }